The broad objective is to study the physiology of the mamalian kidney particular that of the renal medulla. Our investigations will center upon three general topics described below. The experimental approach will be direct, urging primarily micropuncture and related micro-analytical techniques, in normal rodents and two special strains of rats - one with hereditary diabetes insipidus (Brattleboro) and the other with a long renal papilla (Munich-Wistar). Urinary concentrating mechanism. We plan to test the validity of recent passive models of the concentrating mechanism; to study the transtubular and transcapillary determinants of flow across Hene's thin loop and vasa recta in vivo; to draft a mathmatical model of the medullary microcirculation; and to study the kinetics of dluid and urea reabsorption in the collecting tubule system. Nephron heterrgeneity. We plan to determine the glomerular filtration rate of single superficial and juxtamedullary nephrons in the Munich-Wistar rat, to measure post-glomerular capillary plasma protein concentration and to calculate the respective single nephron filtration fractions and glomerular capillary plasma flow rates to the two kinds of nephrons. Calcium reabsorption. We plan to study the effect of parathyroid hormone on calcium reabsorption in the superficial and juxtamedullary nephron in the parathyroidectomized rat. We hope to analyze the constituents in fluid from Henle's loop and vasa recta plasma using the electron probe, and to measure the transtubular electrical potential difference across Henle's thin loop in vivo.